1. Field of the Invention
The present invention relates to a piezoelectric element having an electro-mechanical conversion function and a method for manufacturing the same, and an ink jet head and an ink jet recording apparatus using the piezoelectric element.
2. Description of the Prior Art
Generally, a piezoelectric element has a layered structure in which a piezoelectric material is interposed between two electrodes in the thickness direction. A piezoelectric material is made of a material capable of converting a mechanical energy to an electrical energy and vice versa. Typical examples of a piezoelectric material include lead zirconate titanate (Pb(Zr,Ti)O3) (hereinafter referred to as “PZT”), which is an oxide having a perovskite crystalline structure, and substances obtained by adding magnesium, manganese, nickel, niobium or the like to PZT.
In the case of PZT having a perovskite tetragonal crystalline structure, in particular, the greatest piezoelectric displacement is obtained in the <001> axis direction (i.e., the c axis direction). However, many of the piezoelectric materials are polycrystals made up of a collection of crystal grains, and the crystallographic axes of the crystal grains are oriented randomly. Therefore, the spontaneous polarizations Ps are also arranged randomly. Nevertheless, piezoelectric elements are manufactured so that the sum of these vectors is in a direction parallel with an electric field. In a piezoelectric actuator (formed by providing a vibration plate on the layered structure), which is one form of piezoelectric element, if a voltage is applied across the two electrodes, a mechanical displacement which is proportional to the magnitude of that voltage is obtained.
However, if a high voltage is applied to the piezoelectric element that has been subjected to a high humidity atmosphere for a long time, the electrical insulation of the piezoelectric material is decreased, resulting in a dielectric breakdown. This phenomenon has been one of the major problems in achieving the reliability of the piezoelectric element.
In order to prevent the occurrence of this kind of phenomenon, various measures have been taken so far. In particular, to avoid the occurrence of electrode material migration, which is considered to be most closely related to dielectric breakdown, selecting, as the electrode material, gold or platinum in which migration is not likely to occur, has been considered.
Nevertheless, it has become clear that even if electrode material migration is prevented by using gold or platinum as the electrode material, the insulation resistance of the piezoelectric material decreases. In other words, this insulation resistance decrease results from the fact that water directly attacks the piezoelectric material. As a method for preventing this decrease, it has been proved that if the entire body of a piezoelectric element is placed in a metallic airtight container in which a desiccating agent is provided and then the container is sealed completely, insulation deterioration does not occur in the piezoelectric material (see Japanese Laid-Open Publication No. 4-349675).
Along with the recent downsizing of electronic appliances, there is a strong demand for reducing the size of piezoelectric elements. In order to meet the demand, more piezoelectric elements are used in the form of thin films whose volumes are significantly reduced from those of sinters, which have conventionally been used in various applications, and active researches and developments have been made for reducing the thickness of piezoelectric elements. Examples of PZT film formation methods include a sputtering method, a CVD method, a sol-gel method, or the like. By these methods, piezoelectric thin films with excellent characteristics can be obtained, if special consideration is given to oxygen concentration adjustment or heat treatment conditions.
For the purpose of downsizing, it also is desired that a piezoelectric element is not placed in a metallic airtight container as mentioned above, but is used in an uncovered state. To meet this demand, measures have also been taken to prevent a piezoelectric element from deteriorating even if it is used in a high humid environment without being placed in a metallic container. For example, a method has been proposed in which a heat generating film is provided alongside a piezoelectric layer that forms a piezoelectric element, so that the heat generating film positively heats the piezoelectric layer, thereby preventing the piezoelectric layer from absorbing moisture (see Japanese Laid-Open Publication No. 2000-43259).
Piezoelectric elements which contain a lead compound such as PZT are synthesized at high temperature. This is true for piezoelectric elements in the form of thin films. Considering that the vapor pressure of lead is high at high temperature, a PZT thin film, for example, is typically made to have a composition in which lead is contained excessively to some degree as compared with PZT of stoichiometric composition (whose chemical composition formula is Pb(Zr1-xTix)O3(0<x<1) and chemical composition proportion is Pb:Zr+Ti: O=1:1:3) (see Japanese Laid-Open Publication No. 10-290033, for example).
The present inventors examined why a piezoelectric element, which uses such a piezoelectric thin film containing an excessive amount of lead, has a dielectric breakdown when a high voltage is applied to the piezoelectric element in a high humidity environment, and found out the following mechanism. More specifically, a piezoelectric thin film is often made up of a collection of columnar crystal grains oriented from one side to the other side in the thickness direction of the piezoelectric thin film fabricated by a sputtering method, for example. The boundaries of these columnar crystal grains exist as grain boundaries. Even a piezoelectric thin film, which is not in the form of a columnar crystal grain collection, also has many grain boundaries. The present inventors then found that in the grain boundaries in a piezoelectric material, excessive lead is present in the form of an oxide and that the lead oxide existing in the grain boundaries electrochemically reacts with water content absorbed as moisture and then changes in quality. They considered that the cause of the occurrence of dielectric breakdown in the conventional piezoelectric element was that water content passed through pinholes in the electrode films into the grain boundaries in the piezoelectric thin film to cause lead oxide present in the grain boundaries to react with the water content and change into hydroxide lead and then into lead dioxide, which possesses electrical conductivity.
From this consideration, it is found that this problem can be overcome by eliminating the direct attack by the water content on the piezoelectric material that contains a lead compound such as PZT. To that end, the following method is possible. In fabricating a piezoelectric element, after formation of a layered structure of a piezoelectric material and first and second electrodes, either the first or second electrode is exposed to a chemical substance such as zirconium alkoxide, zirconium acetylacetonate, or zirconium carboxylate so that the chemical substance is absorbed into the piezoelectric thin film through the electrode, whereby lead oxide or hydroxide lead present in the grain boundaries in the piezoelectric thin film is covered by electrochemically stable zirconium oxide, thereby preventing leakage current from passing through the grain boundaries. In this manner, the grain boundaries are covered by the insulating film made of zirconium oxide, such that the electrochemical characteristic of the grain boundaries is controlled by the zirconium oxide existing in the grain boundaries. This allows the grain boundaries to be in an electrochemically stable state, thereby preventing the occurrence of dielectric breakdown in which leakage current passes through the grain boundaries. In this way, the occurrence of dielectric breakdown is prevented even at high humidity.
However, in the above method, since zirconia processing or the like has to be performed after the formation of the layered structure, the fabrication process becomes complicated.